1. Field of the Invention
The present invention relates to an R-T-B based sintered magnet, and particularly relates to a magnet excellent in corrosion resistance.
2. Description of the Related Art
R-T-B based sintered magnets containing a tetragonal R2T14B compound (“R” represents a rare earth element, “T” represents one kind transition metal element of Fe or two or more kind transition metal elements of Fe and other element(s), and “B” represents boron) as the main phase are known to exhibit excellent magnetic properties, and have been a typical high-performance permanent magnet after being invented in 1982 (Patent Document 1).
In particular, R-T-B based sintered magnets whose rare earth element “R” consists of Nd, Pr, Dy, Ho, and Tb have a have large anisotropic magnetic field Ha and have been widely used as a permanent magnet material. Among them, Nd—Fe—B based permanent magnets whose rare earth element “R” is Nd have a good balance in saturation magnetization Is, Curie temperature Tc, and anisotropic magnetic field Ha, and are widely used in consumer, industrial, transport machinery and apparatuses and the like. R-T-B based sintered magnets, however, contain rare earth elements as their main component, and are thus known to have a comparatively low corrosion resistance.
A mechanism of corrosion is thought as follows. First, when water from water vapor or so in use environment adheres on the surface of a sintered magnet, a battery reaction occurs due to a potential difference between a main phase and a grain boundary, and a hydrogen is generated in this process. This generated hydrogen is stored in an R-rich phase, and the R-rich phase is thus changed to a hydroxide. Furthermore, a hydrogen is generated at an amount that is larger than an amount of the hydrogen stored in the R-rich phase due to the battery reaction between the water and the R-rich phase the hydrogen has been stored. The above reaction progress expands a volume of grain boundary portions and causes main phase particles to fall off. As a result, a newly formed surface of the R-T-B based sintered magnet appears, and the above reaction progresses inside.
For this problem, Patent Document 2 discloses that corrosion resistance is improved by forming an R—O—C—N concentrated part where each concentration of “R”, O, C, and N is higher than that of R2T14B crystal grains in grain boundaries at a predetermined rate.
Patent Document 1: JP 59-46008 A
Patent Document 2: JP 5392440 B